2996 - Kilovoltage Intraoperative Radiotherapy or EBRT Boost in Early-Stage High-Risk Breast Cancer: A Matched-Pair Analysis

Purpose/Objective(s):

To evaluate the clinical effectiveness of an upfront intraoperative radiotherapy (IORT) boost against sequential external-beam (EBRT) in a limited-resource setting, before the adoption of simultaneous integrated boost (SIB).

Materials/Methods:

Patients from a large tertiary referral center, treated between 2014 and 2019 were retrospectively assessed and included. The IORT cohort encompassed patients prospectively recruited within an institutional registry, who were treated with a miniaturized linear accelerator with a low-energy x-ray output. A total 202 patients per arm were included in the analysis and matched per T and N stage, age at diagnosis, year of treatment, and systemic treatment features. The analysis was performed using the t-test or the non-parametric Mann-Whitney test. The association of qualitative variables with the treatment received, either IORT or EBRT, was analyzed using the Chi-square test through contingency tables. The condition of having no more than 20% of the cells with expected values less than 5 was verified. If this condition was not met, categories of the qualitative variable were grouped. For 2x2 contingency tables, the Yates correction was applied. Cumulative local control, cancer-specific survival and overall survival were evaluated and reported according to the Kaplan-Meier method.

Results:

Median follow-up was 57,5 and 58 months for IORT and EBRT, respectively. No significant imbalances were detected between both groups. The median age was 51 years old in both cohorts (p=0.987). Patients in both arms underwent similar systemic therapies (hormone therapy 77.1% vs. 76.6%, p=0.906; adjuvant chemotherapy 74.8% vs 75.2%, p=0.909). More patients in the IORT group showed G3 histologies (47.2% vs 30.2%, p=0.004) and in-situ component in the final pathology (41.6% vs 23.2%, p<0.001). Hypofraction was employed in 58.2% and 76.7% of patients (p<0.001). The 5-year cumulative local-recurrence rates were 3.4% and 2.4%, respectively (p=0.37). The cumulative 5-year nodal-recurrence rates were 1.6% and 0.6% (p=0.8), while the distant-metastases rates were 5.5% vs 5.1% (p=0.72). The 5-year estimated cancer-specific survival was 95.4% vs. 98.6% (p=0.72) and estimated overall survival 91.5% vs. 98% (p=0.11) with total 17 and 16 events along the follow-up period.

Conclusion:

No significant differences in terms of disease control or survival were identified between IORT and EBRT boosts for early-stage, high-risk breast cancer patients. In a limited-resources setting, where no SIB is available or is yet to be implemented, IORT poses an interesting alternative to shorten treatment times. Future studies should address the clinical value of IORT boost in the SIB era.